The present invention relates to an active matrix type display device employing light emitting devices such as EL (electro-luminescence) devices or LEDs (light emitting diodes) each of which emits light by causing a driving current to flow through a light emitting thin film such as an organic semiconductor thin film, and thin film transistors for controlling the light emitting operation of the respective light emitting devices.
In recent years, as the advanced information society has come, there has been increasing demands for personal computers, portable information terminals, information communication apparatuses or complex products thereof. A thin and light-weight display device is suitable for these products, and hence the liquid crystal display device or the display device constituted by the self-light emitting type EL devices or the LED devices. The self-light emitting type display device of the latter has the features that the visibility is excellent, the visible angle characteristics are wide, it is suitable for the moving pictures since it is excellent in the high speed response, and so forth, and hence it is expected that the self-light emitting type display device will be important more and more in the information communication field in the future. In actual, recently, the rapid enhancement of the light emitting efficiency of the organic EL device or the organic LED device (hereinafter, the OLED is the general form for these devices) in which the organic material is used as the light emitting layer, and the advance of the network technology for making the image communication possible are combined to make the expectation to the OLED display device go on rising.
An example of the OLED display device according to the prior art is described in Pioneer R&D Vol. 8, No. 3, pp. 41 to 49. In accordance with this example, as shown in FIG. 6A, OLEDs are respectively arranged in the intersections of n anodes 61 which extend longitudinally and m cathodes 62 which extend transversely to form a simple matrix in which pixels P11, . . . , Pmn are provided. Then, each of the anode lines is driven by a constant current voltage-source 63 every cathode line to scan the cathode lines in the line-at-a-time manner. In such a way, the time division driving is carried out. Each of the pixels can be expressed in the form of an equivalent circuit shown in FIG. 6B, in which a parasitic capacity 65 is parasitically connected in parallel with an OLED 64. The value of this parasitic capacity 65 is so large as to be about 20 pF in the square of 0.3 mm×0.3 mm, and hence in order to obtain the desired picture quality by the time division driving requiring the high speed as described above, it is necessary to devise the driving waveform for which the charge and discharge of the electric charges to and from the parasitic capacity are taken into consideration. In actual, in the above-mentioned prior art, there is adopted the complicated driving method wherein the timing in which all of the electrodes are grounded once is provided.
Instead of the above-mentioned simple matrix, the active matrix driving in which TFTs are provided in the pixels, respectively, has also been studied. The technology for manufacturing the OLED display device in the form of the active matrix structure to drive the same, for example, is disclosed in JP-A-8-241048 and U.S. Pat. No. 5,550,066, and also in WO98/36407 in which the contents of the driving voltage are described in more detail. For the typical pixels of the OLED display device of the active matrix system thus disclosed, as shown in FIG. 7, the light emission luminance of the OLED 76 is controlled by the active device driving circuit constituted by at least two TFT switch transistor Tsw73 and driver transistor Tdr74, and one accumulation capacitor 75. More specifically, the voltage corresponding to the electric charges which are accumulated in the accumulation capacitor 75 through the switching transistor 73 provides the gate voltage of the driver transistor 74, and the OLED 76 is driven by the current which is determined on the basis of the gate voltage. However, in actual, there arises the problem that the ununiformity of the display picture quality is generated due to the ununiformity of the threshold voltage and the charge drift mobility of the driver transistor.
As for the system having the possibility of clearing the above-mentioned two problems, as shown in FIG. 8, the active matrix system of providing one transistor in one pixel to carry out the driving is disclosed in JP-A-4-125683.